622 Part VI: Fermented Foods
the absence of oxygen and to the nonlimiting supply
of fermentable sugars (Maloney and Foy 2003).
The amount of maltose available is a complex
interaction between the amount of damaged starch,
the level of amylases in the flour and the stage and
length of the fermentation process. Maltose accu-
mulates during the early stages of the fermentation
because it is generated by amylase but is not metab-
olized by the yeast because the presence of glucose
represses maltose utilization.
When readily fermentable sugars (glucose and
fructose) are exhausted, the yeast shows a lag in fer-
mentation and then turns its metabolism to the malt-
ose produced from the action of -amylase on
starch. If sucrose has been added in the bread formu-
lation (e.g., 4%), this is fermented in preference to
maltose, and the lag in the fermentation may not be
observed. High amounts of added sucrose (e.g.,
20%) significantly retard fermentation due to the
high osmotic stress on the yeast (Maloney and Foy
2003).
The products of yeast metabolism in dough fer-
mentation vary considerably with pH. In bread, the
pH is usually below 6.0, but above this, end products
in addition to ethanol and CO 2 are formed, such as
succinate, acetic acid, and glycerol, and less ethanol
and CO 2 are formed. S. cerevisiaeis also able to
degrade proteins and lipids, and several flavor com-
pounds are produced (Fig. 27.3).
It is generally not considered that the yeast fer-
mentation is important for bread flavor and aroma
development in traditional bread processes. How-
ever, the modern mechanical dough development
processes, where the fermentation stage has been
radically reduced, produce bread with a flavor that is
inferior to that produced by the traditional straight
dough process. This indicates that the yeast fermen-
tation does make a positive contribution to bread fla-
vor (Stear 1990). Zehentbauer and Grosch (1998)
showed that yeast level and fermentation time and
temperature affected aroma in the crust of baguettes,
and they identified the flavor compounds 2-acetyl-1-Figure 27.3.Biochemical changes during yeast fermentation of bread.